Wayne,
It is a 2.4Ghz set so it can be used for any type of model (separate frequencies only applied to the old crystal controlled sets.)
Planet receivers are widely available from model retailers such as Cornwall Model Boats and Howes Models etc. Just Google Planet Radio Control.
Just about all handbooks describe use for model flying purposes as that is where the bulk of the market is. It doesn't mean that you can't use them for boats as well.
Colin